The bands incorporated in such spring interiors have been such that the bridging portion of each interconnecting segment is rectilinear and lies at or near the place where a side face of the band meets the top or bottom face thereof. When bands of springs of that design are incorporated in spring interiors, the bridging portion of each link therefore extends from one helical wire to the next in a direction at right angles to the helical wires.
It will be appreciated from the foregoing description that the top face of a spring interior assembled in this way has the general appearance of a rectangular grid. Each of the transverse elements of the grid comprises a helical wire, and each of the longitudinal elements of the grid comprises a row of mutually aligned bridging portions. Within the confines of each rectangle of the grid and disposed a little lower than the grid are the upper end portions of two adjacent coil springs, those two springs constituting parts of the same band of springs. The bottom face of the spring interior is, of course, similar to the top face, though inverted.
In this description of the invention there are references to faces of bands of springs and of spring interiors. As the bands of springs and spring interiors are, of course, of open-work or skeletal form, the term "face" must be understood as referring to an imaginary surface defined by the relevant parts of the bands or spring interiors. Furthermore, as the wires and helical wires are of finite width or thickness and as they sometimes overlap each other, the term "face" cannot be understood as having a strictly geometrical meaning. Nevertheless, as the faces concerned are relatively extensive and are of flat shape, their locations can in practice be determined without difficulty or ambiguity.
It is customary for a spring interior to be incorporated in an upholstered article. In such an article at least one of the main faces of the spring interior (that is the top and bottom faces thereof) is covered by a layer or layers of padding. This in turn may be covered by a cover made of sheet material such as ticking or upholstery fabric. In use, when pressure is applied to an upholstered face of such an article the filling is pressed towards the spring interior, and in the absence of any additional support tends to enter the rectangular openings in the grid. When the thickness of the filling is relatively small as compared with the dimensions of each rectangular grid openings the upholstery tends to be deformed in such a way that the pattern of the grid forms relatively unyielding ridges in the cover and can be felt by the user. This entry of the filling into the spring interior is a typical example of a phenomenon known in the trade as "cupping."
One method commonly employed to prevent or reduce "cupping" is the provision of a flexible diaphragm or separator between the spring interior and the filling. The diaphragm or separator may comprise a sheet of flexible but inextensible fabric or it may comprise a sheet of mesh. One type of mesh that is widely used comprises a plurality of uniformly spaced parallel cords of twisted paper or plastic strip traversed at right angles by a plurality of uniformly spaced, parallel resilient wires, the wires piercing all the cords except the two marginal cords, and end portions of the wires being shaped to form tight loops or eyes embracing the marginal cords. The rectangular apertures in such a mesh are considerably smaller than those in the grids of the top and bottom faces of the spring interior so that any tendency for "cupping" is much reduced.
British Patent No. 2,143,731 describes a spring interior comprising a plurality of bands of interlocked springs wherein the bands are disposed side by side so that their top faces lie in a top main face of the spring interior and their bottom faces lie in a bottom main face of the spring interior. The bands of springs are interconnected by helical wires lying in the top and bottom faces of the bands and extending across the bands with each helical wire embracing portions of wires of the bands that extend transversely of the bands. The adjacent coils of each band are interconnected by interconnecting segments of the wire from which the band is formed. These interconnecting segments have bridging portions which extend lengthwise of the row of coils of the band. These bridging portions are shaped so as to extend not only longitudinally of the bands but also laterally thereof, the laterally extending portions constituting supporting structures for supporting padding and for preventing "cupping" of the padding. When padding or other upholstery material is disposed against a face containing those supporting structures, there is a reduction in any tendency there might otherwise be for the padding to enter the band through that face when forces are applied to the material in a direction or directions such as to compress the coil springs incorporated in the band.
A characteristic of the spring interior described in British Patent No. 2,143,731, as well as most spring interiors, is that they are of relatively constant firmness throughout the deflection of the springs of the interior. Otherwise expressed, a "firm" spring interior is firm throughout the deflection of the spring interior and a "soft" spring interior is "soft" throughout the deflection of the springs of the interior. But ideally, for many bedding applications, a spring should be "soft" or relatively easily deflected for the first increment, as for example, the first one inch, of deflection for a soft feel and thereafter firm for good body support. But the nature of most springs is that they do not lend themselves to this type of deflection pattern. Accordingly, prior art practice has been to provide a relatively thick, soft pad atop the spring mattress, which pad provides the initial "soft" feel of the resulting padded spring interior.
It has therefore been one objective of this invention to provide a spring interior which has an initial "soft" deflection followed by a much firmer deflection provided by the springs of the interior rather than padding. More specifically, it has been an objective of this invention to provide a spring pattern manufactured in accordance with the description contained in the above-identified British patent, but with an initial soft feel or initial soft deflection of the spring interior provided by the springs and not by a thick and expensive pad covering the spring interior.